Thin film polymeric gel electrolytes

ABSTRACT

Novel hybrid thin film electrolyte, based on an organonitrile solvent system, which are compositionally stable, environmentally safe, can be produced efficiently in large quantity and which, because of their high conductivities ≈10 -3  Ω -1  cm -1  are useful as electrolytes for rechargeable lithium batteries.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under contract no.DE-AC04-94AL8500 awarded by the U.S. Department of Energy to SandiaCorporation. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

This invention pertains generally to electrolytes and particularly tohybrid thin film polymer electrolytes for lithium rechargeablebatteries.

Lithium-based rechargeable batteries offer significant advantages overother rechargeable electrochemical systems in terms of improvedperformance characteristics such as increased power and energy densityand cycle life, particularly for consumer applications such ascomputers, power tools, etc. For this reason, a significant amount ofresearch is aimed at improving the characteristics of rechargeablelithium batteries. Of particular interest, is the development ofimproved electrolytes.

Aprotic liquid electrolytes used in lithium rechargeable batteriespossess the high conductivities necessary for high energy and powerdensity applications, however, they suffer from several significantdisadvantages. Batteries having liquid electrolytes are prone to leakand consequently, present a safety problem; they suffer from anodepassivation which reduces output and life; and because they cannot beproduced by continuous processes, manufacturing is inefficient. Solidelectrolytes overcome most of these disadvantages except that they havelow conductivities at normal operating temperatures (≦10⁻⁵ ohm⁻¹ cm⁻¹),limiting the current that can be withdrawn from the battery.

Hybrid thin film electrolytes, which are basically liquid electrolytesin which the liquid phase has been immobilized by incorporation into apolymer, combine the best features of both liquid and solidelectrolytes; retaining the high conductivity of liquid electrolytes andthe electrolyte immobility of solid electrolytes while overcoming thedisadvantages of both liquid and solid electrolytes. Because of theirattractive features these electrolytes have been studied extensively anddemonstration batteries incorporating hybrid thin film electrolytes havebeen built. However, the ambient temperature conductivities havegenerally been too low to be useful for most battery applications. Priorattempts to fabricate hybrid thin film electrolytes and the use of theseelectrolytes in batteries has been summarized in K. M. Abraham, AmbientTemperature Polymer Electrolyte Batteries, Fourth Annual InternationalRechargeable Battery Seminar, Deerfield Beach, Fla., Mar. 3, 1992 and B.Barnett and D. Fauteux, New Directions in Polymer Electrolyte BatteryTechnology, ibid.

In these hybrid thin film electrolytes, the immobilizing polymer phasecomprises two basic forms; chemically crosslinked polymers and polymersthat can form a physical network, i.e., capable of forming a gel basedon physical interactions among their linear chains. The electrolyte,contained within the polymer phase, generally comprises a lithium saltand an aprotic solvent or mixture of solvents capable of dissolving thelithium salt. Because of their good mechanical properties, these hybridelectrolytes can be cast as thin films that allow ions to move throughthem when a potential is applied between the electrodes on which thefilm is deposited.

It is well known to those skilled in the electrochemical art that theionic conductivity of an electrolyte is strongly dependent upon theconcentration of the solute. Consequently, it is advantageous to raisethe concentration of the solute in the electrolyte until an optimumconcentration is reached, typically that concentration of solute atwhich the solution exhibits a maximum conductivity. Propylene carbonatehas been used as a solvent for lithium salts because of its highdielectric constant. As shown by Abraham et al. (J. Electrochem. Soc.,137, 1657-1658, May 1990), the conductivities of hybrid thin filmelectrolytes having a propylene carbonate solvent can be as high as 10⁻³ohm⁻¹ cm⁻¹ However, propylene carbonate has a high vapor pressurerelative to other high dielectric constant organic solvents, such assuccinonitrile, which makes propylene carbonate less desirable for usein battery applications. Studies of the conductivity of hybridelectrolytes have generally employed LiClO₄ as a solute, e.g., Watanabeet al. (J. Polymer Science, 21, 939-948, 1983). However, because it is astrong oxidizing agent, LiClO₄ is unacceptable as a solute for batteryapplications for safety reasons. What is needed is a hybrid thin filmelectrolyte, useful for lithium rechargeable batteries, having aconductivity ≈10⁻³ ohm⁻¹ cm⁻¹, and in which the liquid phase containedtherein comprises a good solvent for lithium salts, has a low vaporpressure at operating temperatures that would be experienced byrechargeable lithium batteries and a lithium containing solute whichdoes not present safety hazards.

Responsive to these needs, the instant invention discloses novel hybridelectrolyte thin films which are compositionally stable, environmentallysafe, can be efficiently produced in large quantities and which, becauseof their high conductivities ≈10⁻³ ohm⁻¹ cm⁻¹ are useful as electrolytesfor rechargeable lithium batteries.

SUMMARY OF THE INVENTION

The hybrid thin film electrolytes of the instant invention comprise apolymer gel that contains within the polymer gel structure a solutioncomprising ethylene carbonate, an organonitrile and at least one lithiumsalt having a polarizable anion. These hybrid film electrolytes can bemade by dissolving lithium salts and a gel forming polymer in an excessof ethylene carbonate and an organonitrile solvent or a combination oforganonitrile solvents to form a solution. The solution is spread into athin layer, typically by casting onto a plate, such that when part ofthe solvent is removed by evaporation or other means the polymer iscaused to form a clear gel film. In order for the conductivity of thepolymer film to be adequate for use as an electrolyte in a lithiumrechargeable battery, it is desired that the film retain some of thesolution. The hybrid thin film electrolytes made by the method of thepresent invention can have conductivities from about 9×10⁻⁴ to 3×10⁻³ohm⁻¹ cm⁻¹ and thickness of less than 0.5 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the configuration of the cell used to measureconductivities of the hybrid thin film electolytes.

DETAILED DESCRIPTION OF THE INVENTION

The hybrid thin film electrolytes of the present invention are basicallyliquid electrolytes in which the liquid phase has been immobilized byincorporation into a polymer gel. The electrolyte, contained within thepolymer gel phase, comprises a lithium salt and a mixture of ethylenecarbonate and an organonitrile or mixture of organonitriles capable ofdissolving the lithium salt.

Hybrid thin film electrolytes of the instant invention, which can rangein thickness from 0.1-0.3 mm in thickness, can be made by dissolvinglithium salts, having polarizable anions, in an excess of solventcomprising a mixture of ethylene carbonate and an organonitrile or amixture of organonitriles at elevated temperatures (≈110 C), to form afirst solution. An amount of a polymer sufficient to form a solid matrixis dissolved in the first solution, thereby forming a second solutionand the second solution is cast onto a plate. By removing part of thesolvent from the second solution by evaporation or other means, a clearfilm can be formed. It will be appreciated by those skilled in the artthat some minimum level of solution be retained by the film in order toattain conductivities in the range desired. For hybrid thin filmelectrolytes of the present invention, a solvent content of between70-90 wt % is desirable. The polymer gel structure is also critical forthe formation of freestanding films, however, as the concentration ofpolymer becomes higher the conductivity drops, therefore it is necessaryto balance these two competing requirements against each other. It hasbeen found that it is preferable that the polymer concentration be lessthan about 15 wt. %. Preferred aprotic solvents include organonitrilessuch as succinonitrile, malononitrile and glutaronitrile either alone orin combination. The preferred polymers are polyacronitrile (PAN) andpoly(vinylidene fluoride) (PVF2). The preferred lithium salts arelithium trifluoromethylsulfonate and lithiumbis(trifluromethylsulfonyl)imide either alone or in combination. Adetailed procedure useful for preparing the hybrid thin filmelectrolytes disclosed in the instant invention is given below.

In an inert atmosphere, a mixture of 2.5 g of succinonitrile and 2.5 gof ethylene carbonate was heated to 120 C. in a 10 ml screw cap vialusing a temperature controlled aluminum heating block. To this hotmixture was added 0.55 g of lithium trifluoromethylsulfonate. After thesalt has completely dissolved, usually in a matter of few minutes, 0.5 gof polyacrylonitrile (PAN) was added in small increments. When theaddition of PAN was complete, the vial was sealed to prevent loss of theethylene carbonate-succinonitrile solvent during the time the PAN wentslowly into solution (≈48 hrs). After dissolution of the PAN wascomplete, the resulting hot viscous solution was cast onto a glassplate. Partial removal of the excess solvent, in order to initiategelation, was accomplished in either of two ways:

1) heating at 125 C. at ≈20 Torr in a B uchi oven for times ranging from5 to 31 minutes.

2) allowing the casting to stand at room temperature in an inertatmosphere.

The second method of solvent removal is preferred because it allows morecareful control over the gelation process. However, any suitablecombination of temperature and/or vacuum, such as would be obvious toone skilled in the art, can be employed to reduce the solventconcentration and initiate gelation. Following completion of thegelation process, the hybrid thin film electrolyte was removed from theglass casting plate. Hybrid thin films were stored in closed containersto prevent solvent loss.

Because ionic conductivity is a process whereby ions diffuse across thehybrid thin film electrolyte under the influence of an applied potentialgradient, those skilled in the art will appreciate that variations insuch parameters as film thickness, degree of gelation, solvent content,solute content, proportions of the various ingredients, etc., can havean effect on the conductivity of the hybrid thin film electrolytesprepared in the manner set forth in the instant invention. Consequently,measurements were done to determine the effect of these variousprocessing parameters on the conductivity.

FIG. 1 shows the apparatus used to measure the conductivity of hybridthin film electrolytes in an inert atmosphere. The hybrid thin filmelectrolyte 110 is placed between two polished nickel electrodes 120having an area of 0.785 cm<2>. Nickel electrodes 120 are held rigidly ina plastic holder 140 by a series of set screws 145 placed at 120° fromeach other. The thickness of the hybrid thin film electrolyte wasdetermined by measuring the distance between the distal ends of theelectrodes with a micrometer both before and after the hybrid thin filmelectrolyte was placed therebetween. As an aid to reproducibility of themeasurements, notch 130 was machined onto the distal ends of eachelectrode. In this way, the thickness measurements were made at the samelocation for each hybrid thin film electrolyte.

The complex impedance of each hybrid thin film electrolyte was measuredover the frequency range of 15-60,000 Hz by an AC impedance measuringsystem consisting of a Solartron 1255 Frequency Response Analyzer and anEG&G Princeton Applied Research Model 273 Potentiostat/Galvanostat whichwas connected to nickel electrodes 120 by leads 160. These data wereplotted as reactance vs. resistance and the reactance values wereextrapolated to zero in order to obtain the bulk resistance of thehybrid thin film electrolyte. The conductivity of the hybrid thin filmelectrolyte was calculated from the formula

    σ=t/R*A

where

σ=conductivity (ohm⁻¹ cm⁻¹)

R =bulk resistance (ohm) at zero reactance

A =area of the sample (cm²)

t =sample thickness (cm)

In Table 1 are shown the effect of lithium trifluoromethylsulfonateconcentration and the succinonitrile/ethylene carbonate ratio on theconductivity of a hybrid succinonitrile/ethylene carbonate/PAN/lithiumtrifluoromethylsulfonate electrolyte. It can be seen that, for givenhybrid thin film electrolyte thickness and polymer (PAN) concentrationand succinonitrile/ethylene carbonate ratio, the conductivity increaseswith the concentration of the solute lithium trifluoromethylsulfonate.For a given value of lithium trifluoromethylsulfonate concentration theconductivity values were reasonably independent of thesuccinonitrile/ethylene carbonate ratio. The values of conductivityranging from ≈5.2×10⁻⁴ ohm⁻¹ cm⁻¹ at a lithium trifluoromethylsulfonateconcentration of about 1.2 wt % to ≈1.1×10⁻³ ohm⁻¹ cm⁻¹ at 7 wt %lithium trifluoromethylsulfonate.

In Table 2 the same relationships are shown except that here the soluteis lithium bis(trifluromethylsulfonyl)imide. At lower concentrations oflithium bis(trifluromethylsulfonyl)imide (≈1-2 wt %) the conductivitywas approximately the same as that measured for lithiumtrifluoromethylsulfonate. However, as the concentration of lithiumbis(trifluromethylsulfonyl)imide increased the conductivity fell belowthat of lithium trifluoromethylsulfonate for approximately the sameconcentration.

Table 3 presents conductivity data for the hybrid thin film electrolytesuccinonitrile/ethylene carbonate/PAN/lithium trifluoromethylsulfonateat different temperatures and film thicknesses. As would be expected,the conductivity of this film increased as the temperature increased.Ranging from about 1×10⁻³ ohm⁻¹ cm⁻¹ at ≈25 C. to about 3×10⁻³ ohm⁻¹cm⁻¹ at 80 C.

From the foregoing description and example, one skilled in the art canreadily ascertain the essential characteristics of the presentinvention. The description and example is intended to be illustrative ofthe present invention and are not to be construed as limitations orrestrictions thereon, the invention being delineated in the followingclaims.

                  TABLE 1                                                         ______________________________________                                        Effect of lithium trifluoromethylsulfonate concentration                      and SN/EC* ratio on the ionic                                                 conductivity of SN/EC/PAN/LIT hybrid electrolytes                             Final               Film      Film                                            Composition (Wt. %) Thickness Conductivity                                    Film ID                                                                              Solvents LIT     PAN   cm      S cm-1                                  ______________________________________                                        Initial SN/EC Weight ratio, 50/50                                             17     86.7     1.2     12.1  0.02    5.16E-04                                18     84.3     3.7     12    0.017   9.08E-04                                19     85.3     3.9     10.7  0.009   9.21E-04                                20     82.8     5.7     11.5  0.023   1.02E-03                                21     "        "       "     0.022    1.02E-03*                              22     83.1     7       9.9   0.018   1.11E-03                                Initial SN/EC Weight Ratio, 10/90                                             23     76.9     2.1     21    0.022   4.34E-04                                24     78.4     5       16.6  0.018   1.09E-03                                25     76.4     7.9     15.7  0.021   1.05E-03                                26     74.7     10.5    14.8  0.024   1.34E-03                                ______________________________________                                         *Repeat evaluation of same cast film (Film ID 20) taken from a different      location                                                                      *SN/EC  Succinonitrile/Ethylene Carbonate                                     LiT  Lithium Trifluoromethylsulfonate                                         PAN  Polyacrylonitrile                                                   

                  TABLE 2                                                         ______________________________________                                        Conductivity and composition data on SN/EC*/PAN/LiB                           electrolytes                                                                  Final               Film      Film                                            Composition (Wt. %) Thickness Conductivity                                    Film ID                                                                              Solvents LIB     PAN   (cm)    S cm-1                                  ______________________________________                                        Initial SN/EC Weight Ratio, 50/50                                             27     91.8      1.3     7     0.0152   4.57E-04                              28     77.4      8.3     14.5  0.0269   3.12E-04                              Initial SN/EC Weight Ratio, 10/90                                             29     89.1      1.7     9.2   0.022    7.06E-04                              ______________________________________                                         *SN/EC  Succinonitrile/Ethylene Carbonate                                     LiB  Lithium Bis(trifluoromethylsulfonyl)inide                           

                                      TABLE 3                                     __________________________________________________________________________    Conductivity of a SN/EC*/PAN/LiT electrolyte as a function of                 temperature                                                                   Initial SN/EC Weight Ratio, 43/57                                                                Film  Film   Tempera-                                      Final Composition (Wt. %)                                                                        Thickness                                                                           Conductivity*                                                                        ture                                          Film ID                                                                            Solvents                                                                            LiT PAN (cm)  S cm-1 Deg. C.                                       __________________________________________________________________________    30   86.4  4.5 9.1 0.0161                                                                              9.86E-04                                                                             RT                                            31   "     "   "   0.0201                                                                              1.15E-03                                                                             RT                                            32   "     "   "   0.0168                                                                              1.08E-03                                                                             RT                                            33   "     "   "   0.0154                                                                              9.00E-04                                                                             RT                                            34   "     "   "   0.0178                                                                              1.04E-03                                                                             22                                            35   "     "   "   0.0097                                                                              1.29E-03                                                                             46                                            36   "     "   "   0.0076                                                                              1.56E-03                                                                             58                                            37   "     "   "   0.0024                                                                              3.06E-03                                                                             80                                            __________________________________________________________________________     *The average conductivity of two samples evaluated in two fixtures was        1.03E 03 S cm1 plus/minus 1.09E 04 S cm1 (10.6%)                         

We claim:
 1. A hybrid thin film electrolyte, comprising: in combination,a polymer capable of forming a gel, said polymer gel having a solutionretained therein, said solution comprising ethylene carbonate, at leastone organonitrile and at least one lithium salt having a polarizableanion and capable of dissolving in said solution.
 2. The hybrid thinfilm electrolyte of claim 1, wherein said polymer is polyacrylonitrileor poly(vinylidenefluoride).
 3. The hybrid thin film electrolyte ofclaim 1, wherein the concentration of said polymer is less than about 15wt %.
 4. The hybrid thin film electrolyte of claim 1, wherein saidorganonitrile is selected from the group consisting of succinonitrile,malononitrile and glutaronitrile and combinations thereof.
 5. The hybridthin film electrolyte of claim 1, wherein said lithium salt is selectedfrom the group consisting of lithium trifluoromethylsulfonate andlithium bis(trifluromethylsulfonyl)imide and combinations thereof. 6.The hybrid thin film electrolyte of claim 1, wherein said thin film isless than about 0.5 mm thick.